Can you convert a 24 volt battery charger to 12 volts?

You can try to charge two 12 V batteries at the same time . Using a series circuit, place both batteries next one to another, and connect the negative pole of one of them to the positive pole of the other. Then use the remained poles to connect to the charger. The only other way possible, is to change the transformer, but this would be as expensive as buying another charger.